=97. Forms of Electromagnets.= Fig. 95 shows a _straight, or bar electromagnet_. Fig. 96 shows a simple form of _horseshoe electromagnet_. As this form is not easily wound, the coils are generally wound on two separate cores which are then joined by a _yoke_. The yoke merely takes the place of the curved part shown in Fig. 96. In Fig. 97 is shown the ordinary form of horseshoe electromagnet used for all sorts of electrical instruments. (See "Apparatus Book," Chapter IX., for home-made electromagnets.) =98. Yokes and Armatures.= In the horseshoe magnet there are two poles to attract and two to induce. The lines of force pass through the yoke on their way from one core to the other, instead of going through the air. This reduces the resistance to them. If we had no yoke we should simply have two straight electromagnets, and the resistance to the lines of force would be so great that the total strength would be much reduced. Yokes are made of soft iron, as well as the cores and armature. The _armature_, as with permanent horseshoe magnets, is strongly drawn toward the poles. As soon as the current ceases to flow, the attraction also ceases. [Illustration: Fig. 96.] [Illustration: Fig. 97.] [Illustration: Fig. 98.] Beautiful magnetic figures can be made with horseshoe magnets. Fig. 98 shows that the coils must be joined so that the current can pass around the cores in opposite directions to make unlike poles. (See "Study," Exp. 164 to 173.) CHAPTER XII. HOW ELECTRICITY IS GENERATED BY INDUCTION. =99. Electromagnetic Induction.= We have seen that a magnet has the power to act through space and induce another piece of iron or steel to become a magnet. A charge of static electricity can induce a charge upon another conductor. We have now to see how a _current_ of electricity in one conductor can induce a current in another conductor, not in any way connected with the first, and how a magnet and a coil can generate a current. [Illustration: Fig. 99.] [Illustration: Fig. 100.] =100. Current from Magnet and Coil.= If a bar magnet, Fig. 99, be suddenly thrust into a hollow coil of wire, a momentary current of electricity will be generated in the coil. No current passes when the magnet and coil are still; at least one of them must be in motion. Such a current is said to be _induced_, and is an _inverse_ one when the magnet is inserted, and a _direct_ one when the magnet is withdrawn from the coil. =101. Induced Currents and Lines of Force.= Permanent magnets are constantly sending out thousands of lines of force. Fig. 100 shows a bar magnet entering a coil of wire; the number of lines of force is increasing, and the induced current passes in an anti-clockwise direction when looking down into the coil along the lines of force. This produces an indirect current. If an iron core be used in the coil, the induced current will be greatly strengthened. [Illustration: Fig. 101.]